This invention concerns a shoe press for a papermaking machine, and more specifically an arrangement for clamping a blanket (sometimes referred to in the art as “a jacket”) thereon.
A shoe press employs a backing roll and an elongated shoe which has a concave surface which matches the convex surface of the backing roll and cooperates with the roll over a length of the order of 25 cm along the direction of travel of the paper web. This extended nip length is desirable to increase the time during which the paper web is subject to pressure to allow more water to pass from the web into a felt on which it is supported, or respective felts between which it is sandwiched. The shoe, which forms one-half of the nip, is stationary. Without further modification, a paper web passing through the nip formed between the shoe and the backing roll would experience unacceptable rubbing on the non-moving shoe. To overcome this problem, a bearing blanket in the form of a cylindrical tube is slidably disposed over the shoe and around the support shaft on which the shoe is mounted. Friction between the bearing blanket and the shoe is greatly reduced due to the presence of an oil film supplied between the shoe and the inside of the blanket. The backing roll, the paper web and the felt on which it is supported are frictionally engaged and in turn engage the upper surface of the bearing blanket, causing it to slide over the shoe and to rotate about the shoe and its support shaft.
On one type of shoe press the cylindrical bearing blanket is open-ended, but this causes a problem with oil contamination of the web and of surrounding equipment as there is a tendency for lubricating oil disposed between the shoe and the blanket to creep around the edges of the blanket. The solution has been to seal the ends of the blanket to two rotating heads which are mounted on the shoe support shaft.
The cross-section of the blanket at the heads is circular, but the cross-section taken through the nip between the shoe and the backing roll has an appearance somewhat similar to an apple in cross-section (see FIG. 1). Thus, as the blanket moves through the nip, it is elastically deformed. Between the ends of the nip and the ends of the blanket, the material of the blanket forms a compound curve as the blanket transits from the shape of the nip to the circular ends of the blanket where they are attached to the rotating head assemblies. This results in wear on the blanket and it must therefore, be periodically replaced.
A typical papermaking machine of which such a shoe press forms a part may produce over 50,000 m2 of paper an hour. Thus, downtime is costly and the method of clamping the blanket to the rotating head assemblies must be compatible with rapid and precise replacement of the blanket.
Prior art clamping arrangements have been described in U.S. Pat. Nos. 5,700,357, 5,141,601 and 5,098,523. These include a plurality of wedge segments arranged radially outwards of the head and each having an inclined surface; and clamping means mounted onto the head for axial displacement relative thereto so as to cause radial displacement of the wedge segments relative to the head. Portions of the blanket extend between the wedge segments and opposing surface means and are clamped therebetween upon radial displacement of the wedge segments.